Many engineering functions and/or analytical processes require moving a fluid. These tasks are increasingly utilizing systems that rely on one or more fluid channels having cross-sectional dimensions ranging from a few thousand to a few tens of microns across. Moreover, there is a drive to provide large-scale integration of multiple fluid handling features on a single substrate (“chip”) in a manner analogous to that in microelectronics.
Most, if not all, of these applications seek to minimize the non-uniformity present to a greater or lesser extent in all fluid handling systems. This non-uniformity results in unintended and undesirable hydrodynamic dispersion and stagnation.
A primary source of non-uniform fluid flow derives from the drag imposed on the fluid at fluid/channel interfaces, and by secondary effects such as eddying. The use of electrokinetics to transport liquids mitigates these sources of dispersion, but non-uniform flow persists even when using electrokinetic transport due to non-uniform electric fields that arise in the fluid where the flow changes direction, or “turns,” at corners or junctions in a channel manifold. Furthermore, these effects are exaggerated as the aspect ratio of the flow channel decreases such that shallow and wide flow channels, which are the most readily fabricated, are the most affected.
What is needed, therefore, is a method for controlling or eliminating non-uniform fluid flow in flow channels, especially in microchannel systems. For example, a method for reducing or eliminating hydrodynamic dispersion in flow channels is sought. Moreover, there is a need for controlling hydrodynamic dispersion in fluids moving in a fluid system as the flow is turned, split, combined, and/or expanded at junctions, corners, “tees,” or branches in a system manifold.
Two different approaches have been used in previous efforts to minimize the dispersion induced by turns and by contractions at the ends of separation channels. Kopf-Sill, et al., and Parce (U.S. Pat. Nos. 5,842,787 and 5,852,495) teach to reduce dispersion by the use of specific channel geometries. In particular, they recommend channels having large aspect ratios such that the channel depths are much greater than their widths. The smaller channel width helps to reduce the difference in transit time along the inner and outer walls of a turn, thereby reducing dispersion. It is also suggested that dispersion can be reduced by fabrication of turns having a depth along the inner radius that is greater than that along the outer radius, thereby reducing the fluid speed along the inner radius. Griffiths, et al., (U.S. Pat. No. 6,270,641) teach the use of geometry to reduce dispersion, particularly by providing contraction and expansion regions at junctions and corners that reduce the cross-sectional area over some portion of the turn or junction. By carefully designing the geometries of these regions, sample dispersion in turns and junctions is reduced to levels comparable to the effects of ordinary diffusion.